The present invention relates a high voltage series resonant mode power supply (SRMPS) used for charging capacitors. More particularly, the invention relates to apparatus and methods for automatically controlling a capacitor-charging SRMPS in one of two operating modes in order to efficiently provide a highly regulated output voltage over a wide range of capacitive output loads, once a charge voltage is reached, and yet always operate at a fixed frequency above the audible range.
An SRMPS is a particular type of power supply that switches an alternating input voltage across a series resonant circuit so as to excite a series resonant current. The series resonant current is delivered to a primary winding of a power transformer. One or more secondary windings of the power transformer are coupled to respective full-wave rectifier circuits. The full-wave rectifier circuits, in turn, are connected in series so as to apply a single rectified voltage across the capacitive output load. The resonant current delivered to the primary winding of the transformer thus couples power to and charges the load capacitor with a pulsating dc current that has a constant RMS value over time.
Advantageously, the output impedance of an SRMPS is high and the output appears as a constant current source. These characteristics allow the power supply to be short-circuit immune, and further allow the power converter stage to be highly buffered from the output load conditions. Such a power supply is thus ideal for driving capacitive loads, which loads may momentarily appear as a short circuit.
As indicated, an SRMPS contains a resonant circuit. This resonant circuit is placed in series with the primary winding of the power transformer used within the SRMPS. The load capacitor is coupled via rectifier circuits to the secondary windings of the power transformer. The resonant circuit produces primary current in the form of sine waves. The energy delivered to the load capacitor, once a full charge is reached, is normally controlled by varying the frequency of the sine wave delivery period to a frequency that keeps the voltage on the capacitor constant, taking care to ensure that any switching of the primary current occurs only when the sine wave current is at zero. Thus, for load capacitors with low leakage currents, a sine wave "quantum" or "packet" (comprising a single sine wave of electrical current) is delivered to the transformer primary followed by a variable dead time before the next sine wave "quantum" is delivered (low duty cycle). As the load capacitor leakage currents increase, as typically occurs with larger capacitors, the dead time between sine wave quantums decreases, so that more and more sine wave quantums are delivered to the high voltage capacitor per unit of time (moderate duty cycle) to maintain the fully charged voltage regulation. During a charging mode, i.e., while charging the capacitor up to a full charge, the dead time is decreased to near zero, causing essentially a continuous, full-wave rectified sine wave current train to be delivered to the load capacitor (100% duty cycle).
Unfortunately, once the capacitor is charged, the rate at which the sine wave quantums or packets are delivered to the output capacitor for low leakage current is usually in the audible range. This causes a noticeable "whine" in the transformer which is not desirable. Further, because of the fixed resonant frequency of the circuit, and the corresponding fixed amount of energy for each sine wave packet, the ripple on the output capacitor may be excessive, particularly for low capacity values where the SRMPS is operating with only an occasional quantum of energy being delivered to maintain a desired charge on the capacitor.
Hence, what is needed is a way of operating an SRMPS at a fixed frequency well above the audible range regardless of the output power requirements of the power supply once the load capacitor is charged, thereby eliminating any "whine" or other similar noise associated with the transfer of energy quantums through a transformer. However, coupled with such need for high frequency (non-audible) operation is the additional need to maintain the precise charge level on the load capacitor of the SRMPS, regardless of the capacitor leakage current, thereby avoiding excessive ripple on the load capacitor.
The present invention advantageously addresses the above and other needs.